Expanded Memory
EMS Memory
Expanded Memory Specification
EMS (or expanded memory) is a type of memory
available on 8088- and 286-based computers that have
a special memory hardware and a supporting device
driver.  It is also available on 386-based computers
when EMM386.EXE (or other emulator) is installed.

EMS works by taking a "window" of memory addresses
between the 640K and 1MB marks, and "paging" blocks
of memory into that window.
  TECH Help! covers programming aspects of EMS in
detail.  See EMS in the Index.